Bonding a wire to a surface of a semiconductor integrated circuit to make electrical contact to the integrated circuit is well known. Wire bonders are commonly used to accomplish the bonding. The wire bonders are capable of applying an ultrasonic energy and force to the wire. In other words, the wire bonders can apply an ultrasonic energy and force to the wire during the bonding process.
As the semiconductor substrates become thinner and as more fragile materials are used such as III-V materials, which are very brittle, the wire bonds are prone to failure or else the bonding process damages an unacceptable number of semiconductor die. The failure mechanism is a lifting of the wire bond or cracks forming in the semiconductor substrate itself. Cratering was noted on the contact pad of the semiconductor substrate from where the wire bond was lifted. There were also cracks around the edges of the crater. The wire bond failures are sometimes difficult to detect visually or during final electrical test since they seem to appear during temperature cycling tests or in actual use where they undergo normal temperature cycling.
The wire that is used to bond to the semiconductor device is very thin and typically has a rounded tip that is applied to the contact pad on the semiconductor device. A wire bonder is used to apply the thin wire with the rounded tip to the contact pad on the semiconductor device. The wire bonder is capable of applying a slight force, a vibrating frequency to the wire as contact is made to the contact pad, as well as controlling the vibrating displacement. The vibrating frequency causes the ultrasonic bonding. However, during the bonding process, the vibrating wire tends to wear or dig a crater into the contact pad. The cratering is believed to cause the failure.
Some solutions attempted in the past were to use a low vibrating displacement along with a low force or a medium vibrating displacement along with a higher force. However, other problems surfaced, such as poor bonds and low bond shear strength. One attempt to solve the problem proposed a thermocompression bond but this process took too long thereby increasing the manufacturing time. Another proposed solution was to increase the thickness of the bonding pad with the goal of preventing the cratering from causing the semiconductor substrate from cracking underneath the crater where the wire bond was made. But this proposed solution increased cost and also increased the process time.
Accordingly, the present invention overcomes the above problems and produces a reliable wire bond.